A classical function of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is to maintain calcium homeostasis in vertebrate organisms. This activity is achieved through direct actions on intestine, kidney and bone, and feedback regulated at the parathyroid gland. 1,25(OH)2D3 also exerts additional biologic actions on a wide range of tissue types, primarily as a regulator of cell growth and differentiation. These highly pleiotropic actions suggest that 1,25(OH)2D3 or synthetic derivatives thereof may be useful therapeutically for such indications as cancer, and autoimmune and skin diseases. This utility is plagued, however, by the propensity for 1,25(OH)2D3 to hyper-induce intestinal calcium absorption, renal calcium reabsorption and bone calcium resorption. Recently, however, our understanding of these biologic processes has increased substantially, due largely to the discovery of key target genes whose products play central roles in orchestrating the homeostatic events. As a consequence, three specific aims are proposed. Aim 1: To determine the molecular mechanisms that underlie the regulation by 1,25(OH)2D3 of genes that are central to the calcium homeostatic actions of intestine, kidney and bone in vivo. We will use novel molecular techniques to characterize 1,25(OH)2D3`s ability to promote VDR/RXR DNA binding, coactivator interaction, chromatin modification, RNA pol II recruitment, and induction of renal TRPV5, intestinal TRPV6, and skeletal RankL gene activity in a mouse model in vivo. Aim 2: To evaluate the role of intracellular, vitamin D-inactivating metabolism on 1,25(OH)2D3 activity in the three primary organs. We plan to explore the consequence of Cyp24a1 inactivation on 1,25(OH)2D3`s ability to trigger TRPV5, TRPV6 and RankL activation using the Cyp24a1 null mouse. Aim 3: To assess the underlying mechanisms responsible for the increased biological potency and/or altered selectivity manifested by classic vitamin D analogues in vivo. We plan to characterize in vivo the mechanisms responsible for the increased potency, efficacy and selectivity for three well recognized vitamin D analogues. The research proposed herein will provide novel insight into the underlying mechanisms responsible for the calcemic activity of 1,25(OH)2D3 in vivo, define the impact of ligand pharmacodynamics on these activities and identify mechanisms whereby vitamin D analogues exert unique blends of biologic potency, efficacy and selectivity. These concepts will enable more rationale approaches to the design and synthesis of therapeutically relevant vitamin D analogues.